BACKGROUND
[0001] There is increasing need for large amounts of bandwidth to be routed between a ground
based gateway and a spaced based satellite. With the recent announcement of planned
Ka band and Ku band satellite constellations, it would be beneficial if such frequency
band satellite constellations can be used to help satisfy the aforementioned increasing
need for large amounts of bandwidth to be routed between a ground based gateway and
a spaced based satellite.
WO2014/189570 discloses a method for compensating for a non-ideal surface of a reflector in a satellite
communication system which includes the use of a ground based beamformer.
US 2005/100339 discloses a system and method of free-space optical satellite communications. Prabhu
K et al. disclose an analysis of optical modulators for radio over free space optical
communication systems.
BRIEF DESCRIPTION OF THE DRAWINGS
[0002]
Figure 1 is a block diagram describing a wireless communication system, which may
be a satellite communication system.
Figure 2A depicts gateway forward link equipment, according to an embodiment of the
present technology.
Figure 2B depicts components of the ground based beamformer (GBBF) introduced in Figure
2A, according to an embodiment of the present technology.
Figure 3 depicts space segment forward link equipment, according to an embodiment
of the present technology.
Figure 4A depicts a portion of space segment return link equipment, according to alternative
embodiments of the present technology.
Figure 4B depicts a further portion of space segment return link equipment, according
to an embodiment of the present technology.
Figure 5A depicts gateway return link equipment, according to an embodiment of the
present technology.
Figure 5B depicts components of the ground based beamformer (GBBF) introduced in Figure
5A, according to an embodiment of the present technology.
Figure 6 is a high level flow diagram that is used to summarize methods for enabling
a ground based subsystem to produce and transmit an optical feeder uplink beam to
a satellite, according to certain embodiments of the present technology.
Figure 7 is a high level flow diagram that is used to describe additional details
of one of the steps introduced with reference to Figure 6, according to certain embodiments
of the present technology.
DETAILED DESCRIPTION
[0003] Certain embodiments of the present technology described herein relate to system and
sub-system architectures for high throughput satellites (HTS), very high throughput
satellites (VHTS) and very very high throughput satellites (WHTS), which is also known
as ultra high throughput satellites (UHTS), all of which can be collectively referred
to as HTS. Embodiments of the present technology can also be used to implement mobile
satellite services (MSS) and direct-to-home (DTH) satellite services. Because of spectrum
availability, if feeder links between gateway (GW) sites and satellites are at optical
frequencies, then the number of GW sites can be drastically reduced compared to if
the feeder links are at RF frequencies, which leads to significant cost savings in
the space and ground segments. Even with the availability of 5 GHz spectrum at V band
and dual polarization, a satellite with Terabit/sec (Tb/s) capacity would need between
40 and 70 GWs using RF feeder links, depending on the spectral efficiency achieved,
as described in a conference paper titled "
Optical Feederlinks for VHTS - System Perspectives", by Mata-Calvo et al. (Conference:
Proceedings of the Ka and Broadband Communications, Navigation and Earth Observation
Conference 2015. Ka Conference 2015, 12-14 October 2015, Bologna, Italy). In contrast, using optical feeder links can reduce the total active GW count to
one (plus a few sites would be added for diversity and redundancy; but note that V/Q
band or Ka band GWs typically also need diversity and redundancy sites to achieve
high availability).
[0004] Prior to describing details of specific embodiments of the present technology, it
is first useful to describe an exemplary wireless communication system with which
embodiments of the present technology would be useful. An example of such a wireless
communication system will now be described with reference to Figure 1.
[0005] Figure 1 depicts a block diagram of a wireless communications system that includes
a communication platform 100, which may be a satellite located, for example, at a
geostationary or non-geostationary orbital location. In other embodiments, other platforms
may be used such as an unmanned aerial vehicle (UAV) or balloon, or even a ship for
submerged subscribers. In yet another embodiment, the subscribers may be air vehicles
and the platform may be a ship or a truck where the "uplink" and "downlink" in the
following paragraphs are reversed in geometric relations. Platform 100 may be communicatively
coupled to at least one gateway (GW) 105 and a plurality of subscriber terminals ST
(including subscriber terminals 107). The term subscriber terminals may be used to
refer to a single subscriber terminal or multiple subscriber terminals. A subscriber
terminal ST is adapted for communication with the wireless communication platform
100, which as noted above, may be a satellite. Subscriber terminals may include fixed
and mobile subscriber terminals including, but not limited to, a cellular telephone,
a wireless handset, a wireless modem, a data transceiver, a paging or position determination
receiver, or mobile radio-telephone, or a headend of an isolated local network. A
subscriber terminal may be hand-held, portable (including vehicle-mounted installations
for cars, trucks, boats, trains, planes, etc.) or fixed as desired. A subscriber terminal
may be referred to as a wireless communication device, a mobile station, a mobile
wireless unit, a user, a subscriber, or a mobile. Where the communication platform
of a wireless communication system is a satellite, the wireless communication system
can be referred to more specifically as a satellite communication system. For the
remainder of this description, unless stated otherwise, it is assumed that the communication
platform 100 is a satellite. Accordingly, platform 100 will often be referred to as
satellite 100, and the wireless communication system will often be referred to as
a satellite communication system.
[0006] In one embodiment, satellite 100 comprises a bus (e.g., spacecraft) and one or more
payloads (e.g., the communication payload). The satellite will also include multiple
power sources, such as batteries, solar panels, and one or more propulsion systems,
for operating the bus and the payload.
[0007] The at least one gateway 105 may be coupled to a network 140 such as, for example,
the Internet, terrestrial public switched telephone network, mobile telephone network,
or a private server network, etc. Gateway 105 and the satellite (or platform) 100
communicate over a feeder beam 102, which has both a feeder uplink 102u and a feeder
downlink 102d. In certain embodiments, a feeder downlink beam 102d is a spot beam
to illuminate a region 104 on the Earth's surface (or another surface). Gateway 105
is located in region 104 and communicates with satellite 100 via feeder beam 102.
Although a single gateway is shown, some implementations will include many gateways,
such as five, ten, or more. One embodiment includes only one gateway. Each gateway
may utilize its own feeder beam, although more than one gateway can be positioned
within a feeder beam. In one embodiment, a gateway is located in the same spot beam
as one or more subscriber terminals.
[0008] Subscriber terminals ST and satellite 100 communicate over service beams, which are
also known as user beams. For example, Figure 1 shows service beams 106, 110, 114
and 118 for illuminating regions 108, 112, 116 and 120, respectively. In many embodiments,
the communication system will include more than four service beams (e.g., sixty, one
hundred, etc.). Each of the service beams have an uplink (106u, 110u, 114u, 118u)
and a downlink (106d, 110d, 114d, 118d) for communication between subscriber terminals
ST and satellite 100. Although Figure 1 only shows two subscriber terminals within
each region 108, 112, 116 and 120, a typical system may have thousands of subscriber
terminals within each region.
[0009] In one embodiment, communication within the system of Figure 1 follows a nominal
roundtrip direction whereby data is received by gateway 105 from network 140 (e.g.,
the Internet) and transmitted over the forward path 101 to a set of subscriber terminals
ST. In one example, communication over the forward path 101 comprises transmitting
the data from gateway 105 to satellite 100 via uplink 102u of feeder beam 102, through
a first signal path on satellite 100, and from satellite 100 to one or more subscriber
terminals ST via downlink 106d of service beam 106. An uplink (e.g., 102u) of a feeder
beam (e.g., 102) can also be referred to more succinctly as a feeder uplink beam,
and the downlink (e.g., 106d) of a service beam (e.g., a 106) can also be referred
to more succinctly as a service downlink beam. Although the above example mentions
service beam 106, the example could have used other service beams.
[0010] Data can also be sent from the subscriber terminals STs over the return path 103
to gateway 105. In one example, communication over the return path comprises transmitting
the data from a subscriber terminal (e.g., subscriber terminal 107 in service beam
106) to satellite 100 via uplink 106u of service beam 106, through a second signal
path on satellite 100, and from satellite 100 to gateway 105 via downlink 102d of
feeder beam 102. An uplink (e.g., 106u) of a service beam (e.g., 106) can also be
referred to more succinctly as a service uplink beam, and the downlink 102d of feeder
beam 102 can also be referred to more succinctly as a feeder downlink beam. Although
the above example uses service beam 106, the example could have used any service beam.
[0011] Figure 1 also shows a Network Control Center (NCC) 130, which can include an antenna
and modem for communicating with satellite 100, as well as one or more processors
and data storage units. Network Control Center 130 provides commands to control and
operate satellite 100. Network Control Center 130 may also provide commands to any
of the gateways and/or subscriber terminals.
[0012] Figure 1 also shows calibration and pointing stations 150 that are used to determine
amplitude and phase errors associated with forward path and return path signals 101
and 103, which amplitude and phase errors can be used by a ground based beam former
(GBBF) (e.g., 230 in FIGS. 2A and 2B) to perform ground based beamforming, in accordance
with certain embodiments of the present technology. More specifically, the amplitude
and phase errors can be used as, or used to determine, amplitude and phase coefficients
that are used by the GBBF 230 to perform ground based beamforming. In accordance with
certain embodiments, the calibration and pointing stations 150 are part of a calibration
subsystem. Such a calibration subsystem can also include one or more processors and
data storage units. The calibration subsystem can control the transmission and reception
of calibration signals, and can control the execution of algorithms and/or the like
that are used to determine amplitude and phase errors and/or coefficients. The calibration
subsystem may also be used for forward uplink power control and to correct for Doppler
effects, but is not limited thereto.
[0013] In one embodiment, communication platform 100 implements the technology described
below. In other embodiments, the technology described below is implemented on a different
platform (or different type of satellite) in a different communication system. For
examples, the communication platform can alternatively be a UAV or balloon, but is
not limited thereto.
[0014] The architecture of Figure 1 is provided by way of example and not limitation. Embodiments
of the disclosed technology may be practiced using numerous alternative implementations.
[0015] Conventionally, a gateway (e.g., gateway 105) communicates with a satellite (e.g.,
satellite 100) using an antenna on the ground that transmits and receives RF (radiofrequency)
signals to and from an antenna on the satellite. Certain embodiments of the present
technology utilize optical components (instead of antennas) to transmit and receive
optical signals (instead of RF signals) between a gateway and a satellite, as will
be described in additional details below.
[0016] Certain embodiments of the present technology involve the use of analog-over free-space
optical signals, which leads to an elegant architecture for a satellite repeater,
whereby all frequency down-conversion in the forward link is eliminated. An advantage
of this approach, especially for HTS satellites, is that it eliminates the need for
very high speed Analog-to-Digital Converters (ADCs) and Digital to Analog Converters
(DACs) on the satellites. Further, this approach allows the aggregation of multiple
user links but does not require extra hardware associated with an onboard demodulator
and remodulator, and thus reduces the mass, power and cost of the satellite, perhaps
making the difference between being able to launch or not being able to launch the
satellite. In addition, in accordance with specific embodiments where the uplink and
downlink communication signals are modulated at transmit (forward) and receive (return)
RF frequencies, no frequency conversion in the forward link is required on the satellite,
thereby further simplifying the payload design. By contrast, previously envisioned
free-space optical spacecraft architectures proposed demodulation of the optical signal,
followed by routing to user link pathways and remodulation of the signal on user link
RF frequencies. Further, certain embodiments of the present technology eliminate the
need for a satellite to include an onboard channelizer, as will be described in additional
detail below.
[0017] Block diagrams for the communications subsystems for the ground and space segments,
according to certain embodiments of the present technology, are described below with
reference to Figures 2A, 2B, 3, 4A, 4B, 5A and 5B. Certain embodiments use analog
modulation and demodulation on the satellite, thus enabling optical feeder links without
onboard processing.
[0018] Figures 2A and 2B will first be used to describe gateway forward link equipment according
to certain embodiments of the present technology. Figure 3 will then be used to describe
space segment forward link equipment according to an embodiment of the present technology.
In specific embodiments, two hundred and fifty laser wavelengths are combined at a
single gateway (which can be referred to as an optical gateway) and sent to the satellite,
which has multiple (e.g., two hundred and fifty) user beams (also known as service
beams) operating at Ka band frequencies. In accordance with an embodiment, each wavelength
carries 2.5 GHz so that a total of 625 GHz is sent from the gateway on the ground
to the satellite. At a modest spectral efficiency of 2 bps/ Hz, this leads to a 1.25
Tb/s satellite design. In accordance with another embodiment, each wavelength carries
2.9 GHz so that a total of 725 GHz is sent from the gateway on the ground to the satellite.
At a modest spectral efficiency of 2 bps/ Hz, this leads to a 1.45 Tb/s satellite
design. Figures 4A and 4B and Figures 5A and 5B will thereafter be used to depict
return link equipment for a satellite and a gateway.
Gateway Forward Link Equipment
[0019] Figure 2A will now be used to describe gateway forward link equipment 200, according
to an embodiment of the present technology. Such gateway forward link equipment 200
can also be referred to as an optical gateway forward link subsystem 200, or more
generally, as an optical communication subsystem. Referring to Figure 2A, the optical
gateway forward link subsystem 200 is shown as including two hundred and fifty lasers
202_1 to 202_250, two hundred and fifty electro-optical modulator (EOMs) 204_1 to
204_250, a wavelength-division multiplexing (WDM) multiplexer (MUX) 206, an optical
amplifier (OA) 208, and transmitter optics 210. The optical gateway forward link subsystem
200 is also shown as including a user data to spot beam controller 220 and a ground
based beam former (GBBF) 230. The optical gateway forward link subsystem 200 is also
shown as including two hundred and fifty local oscillators (LOs) 242_1 to 242_250,
two hundred and fifty frequency up converters (FUCs) 244_1 to 244_250, and two hundred
and fifty filters (FTRs) 246_1 to 246_250. Each of these elements is described below.
[0020] The user data to spot beam controller 220 is shown as receiving user data signals,
e.g., 10,000 user data signals. An individual user data signal can be for forwarding
to an individual service terminal ST, or multiple user data signals can be for simultaneously
forwarding to an individual service terminal ST. In a broadcast scheme, an individual
user data signal can be for forwarding to multiple service terminals ST simultaneously.
Additional and/or alternative variations are also possible. These user data signals,
as will be described below, are included within an optical feeder uplink beam (e.g.,
102u) that is transmitted by the gateway forward link equipment 200 to a satellite
(e.g., 100), and the satellite includes the user data signals within spot beams (e.g.,
the RF service downlink beams 106d, 110d, 114d and 118d in Figure 1) that are transmitted
to service terminals ST. Assume, for example, that the satellite (e.g., 100 in Figure
1) is configured to transmit one thousand spot beams using two hundred and fifty feed
elements (e.g., feed elements 326_1 to 326_250 in Figure 3), and that the user data
to spot beam controller 220 receives ten thousand user data signals. Continuing with
this example, the user data to spot beam controller 220 would map the ten thousand
user data signals to one thousand spot beam signals, which are provided to the GBBF
230. The one thousand spot beam signals that are provided to the GBBF 230, will, after
they are included within an optical feeder uplink beam (e.g., 102u) that is transmitted
by the gateway forward link equipment 200 to a satellite (e.g., 100), be used by the
satellite (e.g., 100) to transmit one thousand spot beams that each cover a limited
geographic region on Earth. More generally, the user data to spot beam controller
220 is configured to map, to each of a plurality of spot beam signals (e.g., to each
of one thousand spot beam signals), a subset (e.g., ∼ ten) of the plurality of user
data signal signals (e.g., ten thousand user data signals). The user data to spot
beam controller 220, in accordance with certain embodiments, is performed digitally.
[0021] The GBBF 230, as will be described in additional detail with reference to Figure
2B, receives the one thousand spot beam signals, and uses calibration information
received from a calibration subsystem, to produce two hundred and fifty baseband feed
element signals. The two hundred and fifty baseband feed element signals are provided
to respective frequency up-converters (FUCs) 244_1 to 244_250, each of which also
receives an RF carrier signal from a respective one of the local oscillators (LOs)
242_1 to 242_250. In other words, the local oscillators (LOs) 242_1 to 242_250, which
can be referred to collectively as LOs 242, or individually as an LO 242, provide
RF carrier signals to the FUCs 244, so that the FUCs 244 can frequency up-convert
the feed element signals to a desired frequency range. In accordance with certain
embodiments, in order to eliminate a need for RF frequency down-converters in the
forward link equipment (e.g., 300 in Figure 3) onboard the satellite, the carrier
frequencies of the RF signals are used to up convert the feed element signals to the
desired user downlink frequency band within the Ka band (or some other allotted band).
As a result, the satellite repeater is greatly simplified. Still referring to Figure
2A, the two hundred and fifty frequency up-converted feed elements signals (which
can also be referred to more succinctly as feed element signals) are shown as being
filtered by responsive FTRs 246_1 to 246_2, to filter out unwanted frequency components
(e.g., unwanted mixed products) that result from the frequency up-conversions.
[0022] For an example, a portion of the Ka band that may be desirable to use for transmitting
service downlink beams (also referred to as downlink user beams, or spot beams) from
satellite 100 to service terminals ST can be from 17.7 - 20.2 GHz, and thus, includes
a 2.5 GHz bandwidth. In such a case, each of the EOMs 204 could modulate the optical
signal it receives (e.g., via an optical fiber from a respective laser 202) with a
separate RF signal having a frequency within the range from 17.7 - 20.2 GHz. In other
words, the FUCs 244 can be used to frequency up-convert baseband feed element signals
to be within the frequency range from 17.7 - 20.2 GHz. Further, since each of the
two hundred and fifty optical data signals (produced by the two hundred and fifty
EOMs) has a bandwidth of 2.5 GHz, the bandwidth of the optical feeder uplink beam
that is sent from the ground to the satellite is 625 GHz (i.e., 2.5 GHz * 250 = 625
GHz). For another example, a portion of the Ka band that may be desirable to use for
transmitting service downlink beams (also referred to as downlink user beams or spot
beams) from satellite 100 to service terminals ST can be from 17.3 - 20.2 GHz, and
thus, includes a 2.9 GHz bandwidth. In such a case, each of the EOMs 204 could modulate
the optical signal it receives (e.g., via an optical fiber from a respective laser
202) with a separate RF signal having a frequency within the range from 17.3 - 20.2
GHz. In other words, the FUCs 244 can be used to frequency up-convert baseband feed
element signals to be within the frequency range from 17.3 - 20.2 GHz. Further, since
each of the two hundred and fifty optical data signals (produced by the two hundred
and fifty EOMs) has a bandwidth of 2.9 GHz, the bandwidth of the optical feeder uplink
beam that is sent from the ground to the satellite is 725 GHz (i.e., 2.9 GHz * 250
= 725 GHz).
[0023] Still referring to Figure 2A, each of the two hundred and fifty separate EOMs 204_1
to 204_250 is shown as receiving a respective one of the frequency up-converted feed
element signals and a respective one of a plurality of optical carrier signals output
by the two hundred and fifty lasers 202_1 to 201_250, which can be referred to individually
as a laser 202, or collectively as the lasers 202. Explained another way, the light
emitted by each of the two hundred and fifty lasers 202, which can be referred to
as an optical carrier signal, is provided (e.g., via a respective optical fiber) to
a respective one of the two hundred and fifty separate EOMs 204_1 to 204_250.
[0024] The two hundred and fifty separate lasers 202_1 to 202_250 each emit light of a different
wavelength within a specified wavelength range that is for use in producing the optical
feeder uplink beam (e.g., 102u). The lasers as noted above can be referred to individually
as a laser 202, or collectively as the lasers 202. Where the specified wavelength
range is, for example, from 1510 nanometers (nm) to 1560 nm, then the laser 202_1
may emit light having a peak wavelength of 1510 nm, the laser 202_2 may emit light
having a peak wavelength of 1510.2 nm, the laser 202_3 (not shown) may emit light
having a peak wavelength of 1510.4 nm, ... the laser 202_249 (not shown) may emit
light having a peak wavelength of 1559.8 nm, and the laser 202_250 may emit light
having a peak wavelength of 1560 nm. In other words, the peak wavelengths emitted
by the lasers 202 can occur at 0.2 nm intervals from 1510 nm to 1560 nm. The wavelength
range from 1510 nm to 1560 nm, which is within the infrared (IR) spectrum, is practical
to use because IR lasers for use in communications are readily available. However,
wider or narrow wavelength ranges, within the same or other parts of the optical spectrum,
may alternatively be used. For example, it would also be possible to utilize a wavelength
range within the 400 nm - 700 nm visible spectrum. It is also possible that the wavelength
range that is specified for use in producing the optical feeder uplink beam (e.g.,
102u) is non-contiguous. For example, the wavelength range that is for use in producing
the optical feeder uplink beam can be from 1510 nm to 1534.8 nm and from 1540.2 nm
to 1564.8 nm. Further, it is also possible that gateway forward link equipment can
alternatively include more or less than two hundred and fifty lasers (that each emit
light of a different peak wavelength within a specified contiguous or non-contiguous
wavelength range). Additionally, it is noted that the gateway forward link equipment
may include two or more of each of the lasers (that each emit light of a different
peak wavelength within a specified contiguous or non-contiguous wavelength range)
to provide for redundancy or backup. Each of the lasers 202 can be, for example, a
diode-pumped infrared neodymium laser, although the use of other types of lasers are
also within the scope of the embodiments described herein.
[0025] To reduce and preferably avoid interference, the wavelength range that is for use
in producing the optical feeder uplink beam (e.g., 102u) should be different than
the wavelength range that is for use in producing the optical feeder downlink beam
(e.g., 102d). For example, if the wavelength range that is for use in producing the
optical feeder uplink beam 102u is from 1510 nm to 1560 nm, then the wavelength range
that is for use in producing the optical feeder downlink beam 102d can be from 1560.2
nm to 1575 nm. For another example, if the wavelength range that is for use in producing
the optical feeder uplink beam 102u is from 1510 nm to 1534.8 nm and from 1540.2 nm
to 1564.8 nm, then the wavelength range that is for use in producing the optical feeder
downlink beam 102d can be from 1535 nm to 1540 nm and from 1565 nm to 1575 nm. These
are just a few examples, which are not intended to be all encompassing. Details of
how an optical feeder downlink beam (e.g., 102d) can be produced in accordance with
an embodiment of the present technology are provided below in the discussion of Figures
4A and 4B.
[0026] Still referring to Figure 2A, the light emitted by each of the two hundred and fifty
lasers 202, which can be referred to as an optical carrier signal, is provided (e.g.,
via a respective optical fiber) to a respective one of the two hundred and fifty separate
EOMs 204_1 to 204_250. Each of the EOMs is an optical device in which a signal-controlled
element exhibiting an electro-optic effect is used to modulate a respective beam of
light. The modulation performed by the EOMs 204 may be imposed on the phase, frequency,
amplitude, or polarization of a beam of light, or any combination thereof. In accordance
with a specific embodiment, each of the EOMs 204 is a phase modulating EOM that is
used as an amplitude modulator by using a Mach-Zehnder interferometer. In other words,
each of the EOMs 204 can be implemented as a Mach-Zehnder modulator (MZM), which can
be a Lithium Niobate Mach-Zehnder modulator, but is not limited thereto. In accordance
with specific embodiments, each of the EOMs 204 is implemented as an MZM that produces
an amplitude modulated (AM) optical waveform with a modulation index between 10% and
80% in order to maintain fidelity of an RF waveform (modulated therein) without too
much distortion. The optical signal that is output by each of the EOMs 204 can be
referred to as an optical frequency up-converted feed element signal, or more succinctly
as an optical feed element signal. The modulation scheme that is implemented by the
EOMs 204 can result in double- or vestigial-sidebands, including both an upper sideband
(USB) and a lower sideband (LSB). Alternatively single-sideband modulation (SSB) can
be utilized to increase bandwidth and transmission power efficiency.
[0027] The two hundred and fifty separate optical feed element signals that are output by
the two hundred and fifty EOMs 204 are provided to the WDM MUX 206, which can also
be referred to as a dense wavelength division multiplexing (DWDM) MUX. The WMD MUX
206 multiplexes (i.e., combines) the two hundred and fifty optical feed element signals,
received from the two hundred and fifty EOMs 204, onto a single optical fiber, with
each of the two hundred and fifty separate optical feed element signals being carried
at the same time on its own separate optical wavelength within the range from 1510
nm to 1560 nm (or some other contiguous or non-contiguous wavelength range). For example,
as explained above, the two hundred and fifty separate optical feed element signals
can have peak wavelengths of 1510 nm, 1510.2 nm, 1510.4 nm ... 1559.8 nm and 1560
nm.
[0028] The signal that is output by the WMD MUX 206, which can be referred to as a wavelength
division multiplexed optical signal, is provided to the optical amplifier (OA) 208.
The OA 208 amplifies the wavelength division multiplexed optical signal so that the
wavelength division multiplexed optical signal has sufficient power to enable transmission
thereof from the ground to the satellite 100 in space. An exemplary type of OA 208
that can be used is an erbium-doped fiber amplifier (EDFA). However embodiments of
the present technology are not limited to use with an EDFA. The output of the OA 208
can be referred to as an optically amplified wavelength division multiplexed optical
signal.
[0029] The optically amplified wavelength division multiplexed optical signal, which is
output by the OA 208, is provided (e.g., via an optical fiber) to the transmitter
optics 210. The transmitter optics 210, which can also be referred to as a telescope,
can includes optical elements such as lenses, mirrors, reflectors, filters and/or
the like. The transmitter optics 210 outputs a collimated optical feeder uplink beam
that is aimed at a satellite. A gimbal, and/or the like, can be used to control the
steering of the transmitter optics 210. In accordance with an embodiment, the collimated
optical feeder uplink beam has an aperture of about 100 cm, and a half beam divergence
of about 0.0000004 radians, wherein the term "about" as used herein means +/- 10 percent
of a specified value. The use of other apertures and half beam divergence values are
also within the scope of the embodiments described herein. The collimated optical
feeder uplink beam, which is output by the transmitter optics 210, is transmitted
in free-space to receiver optics on a satellite. The term "free-space" means air,
outer space, vacuum, or something similar (which is in contrast to using solids such
as optical fiber cable, an optical waveguide or an optical transmission line). Reception
and processing of the optical feeder uplink beam received at the satellite will be
described in additional detail below. However, before describing the reception and
processing of the optical feeder uplink beam received at the satellite, additional
details of the gateway forward link equipment, according to certain embodiments of
the present technology, will first be provided.
[0030] In order to wavelength division multiplex two hundred and fifty wavelengths produced
by the two hundred and fifty lasers 202_1 to 202_250, a combination of C band optical
frequencies (from 1530 nm to 1565 nm) and L band optical frequencies (from 1565 nm
to 1625 nm) may be used, in order to keep the separation of the wavelengths to be
at least 20 - 25 GHz in order to reduce and preferably minimize inter-wavelength interference
that may occur in an optical fiber due to non-linearities. If fewer wavelengths are
used (e.g., at C band alone), and higher bandwidth is available at Ka band per user
beam (e.g., if 2.9 GHz is available as it is in certain ITU Regions), the overall
throughput still remains of the order of several hundred GHz, which lets the capacity
reach the Tb/s range. If instead each wavelength carries more than the Ka band user
bandwidth, fewer wavelengths can be used, but some amount of frequency conversion
may be needed in the space segment forward link equipment.
[0031] A technology that is increasingly being deployed for use with satellite communication
is ground based beamforming, where a feederlink is segmented into smaller frequency
bands and routed to different feeds that then form beams as needed using ground based
beamforming in a dynamic manner, which allows flexibility to meet changing an evolving
traffic demands. However, at high frequencies (such as Ku band or Ka band) ground
based beamforming typically requires a large number of feeds to form good quality
beams and the per-feed bandwidth multiplied by the number of feeds gets too large
for any RF spectrum to handle. For this reason, ground based beamforming has been
limited to mobile-satellite service (MSS) systems where per-beam bandwidth is limited
and the number of feed elements is also small.
[0032] Embodiments of the present technology use analog over free space optics (AoFSO) technology
to generate the feeder links from a gateway to/from a satellite, thereby using optical
signals to replace the normal Ku or Ka or V band RF spectrum. By modulating these
optical wavelengths at the desired RF frequencies, it is possible to use ground based
beamforming, even at high frequencies like Ka band, and with large numbers of feed
elements, due to the high RF bandwidth available at optical frequencies.
[0033] Beneficially, with ground based beamforming, spot beams can be added, removed or
reconfigured within a gateway to enable a satellite to operate from different orbital
locations and to adapt to changes in traffic patterns or to new applications.
[0034] Figure 2B will now be used to provided details of the GBBF 230 introduced in Figure
2A, according to certain embodiments of the present technology. Referring to Figure
2B, the GBBF 230 is shown as including a GBBF controller 232 and one thousand 1 to
N splitters 234_1 to 234_1000, where N can equal, e.g., two hundred and fifty, but
is not limited thereto. Each individual splitter 234_1 to 234_1000 (which can be referred
to collectively as the splitters 234, or individually as a splitter 234) outputs N
copies of the spot beam signal received by the splitter 234. For example, the splitter
234_1, which receives the spot beam signal_1, outputs N copies of the spot beam signal_1.
The N outputs of each of the splitters 234, are provided to respective phase and amplitude
weight elements 236, which can be implemented in hardware, but are more likely implemented
in software and/or firmware. In accordance with certain embodiments, the function
of the splitters 234 is performed using a digital signal processor (DSP) instead of
N separate splitters. In other words, a DSP can perform the signal copying or replication.
An output of each of the phase and amplitude weight elements 236 is provided to one
of the two hundred and fifty summers 238_1 to 238_250. The outputs of the summers
238_1 to 238_250 are the baseband feed element signals that are frequency up-converted
by the FUCs 244, filtered by the filters 246, and then provided to the EOMs 204, as
shown in Figure 2A, which was discussed above. The GBBF 230 can be implemented entirely
in software. Alternatively, or additionally, the GBBF 230 can be implemented in hardware
and/or firmware. The outputs of the phase and amplitude weight elements 236 can be
referred to a component element signals. The outputs of the summers 238, which outputs
are referred to as the baseband feed element signals above, can also be referred to
as composite signals, since they are a composite of a plurality of component element
signals. In the manner described above, these composite signals are frequency up-converted,
filtered, electro-optically modulated, WDM multiplexed, amplified and optically transmitted
to a satellite.
[0035] The splitters 234 and the weight elements 236 can be collectively referred to as
a signal replication and forward beamforming weighting unit 233. Assuming there are
one thousand spot beam signals provided to the signal replication and forward beamforming
weighting unit 233, and two hundred and fifty feed elements on a satellite (to which
the gateway forward link equipment 200 is transmitting a feeder uplink beam, e.g.,
102u), then the GBBF controller 232 uses calibration signals (received from a calibration
subsystem) to derive element specific amplitude and phase corrections that are applied
individually to two hundred and fifty thousand component element signals (i.e., one
thousand spot beam signals * two hundred and fifty feed elements = two hundred and
fifty thousand component element signals). As noted above, a DSP can perform the signal
copying or replication performed by the splitters 234. More generally, all of the
functions of the elements described within the block labeled GBBF 230 can be implemented
by a DSP. In other words, the GBBF 230 can be entirely or substantially entirely implemented
using a DSP. Nevertheless, it is useful to shown and describe the elements shown in
FIG. 2B in order to understand the operation of such a DSP.
[0036] The calibration subsystem from which the the GBBF controller 232 receives calibration
signals can transmit and/or receive calibration signals upon which calibration measurements
can be performed. These forward and return link measurements can be generally referred
to as a beamforming calibration process, and can be used to initialize, update and
refine the performance of RF service downlink beams (e.g., 106d, 110d, 114d and 118d
in Figure 1) and RF service uplink beams (e.g., 106u, 110u, 114u, 118u in Figure 1).
The calibration and pointing stations 150 in Figure 1 can be part of such a calibration
subsystem. To form a specific beam, the amplitude and phase weightings should be set
to the appropriate values for each feed element, and should be effectively applied
at the feed element apertures. For example, take two feed elements and assume a desired
beam is formed with an amplitude of A1 and a phase of θ1 degrees for feed element
one and an amplitude of A2 and a phase of θ2 degrees for feed element two. In typical
on board beamforming the beam forming operation is very close to the feed aperture,
so it is much simpler to set these values correctly. However, with ground based beamforming,
the signals traverse down independent paths, through the propagation media at different
frequencies which may have differing amplitude and phase channels, and then through
independent conversion paths. One of the paths may experience more amplitude attenuation
and phase shift than the other. Without knowledge and compensation of this difference,
the beam forming weights at the aperture will not be the desired values. Depending
on the error experienced, the desired beam may be mispointed, misshaped, or even dispersed
so grossly as to not be recognizable as a spot beam. However, if the value of the
amplitude and phase difference between the element paths between the feed element
aperture and the ground based beamforming operation is known, it is relatively simply
to be compensated for by adjusting the feed coefficient weights or compensating for
the shift before applying the feed weights. Consequently, in order for the GBBF 230
to function properly a calibration scheme can be used by a calibration subsystem to
determine and compensate for the amplitude and phase variations between the feed element
paths. Examples of such calibration subsystems and schemes for use with ground based
beamforming are described in
U.S. Patent No. 7,787,819 to Walker et al., entitled "Ground-Based Beamforming for Satellite Communications Systems" and in
an article entitled "
Architecture, Implementation and Performance of Ground-Based Beam Forming in the DBSD
G1 Mobile Satellite System" by Walker et al. (28th AIAA International Communications
Satellite Systems Conference (ICSSC-2010)), each of which is incorporated herein by reference.
Space Segment Forward Link Equipment
[0037] Figure 3 will now be used to describe space segment forward link equipment 300 according
to an embodiment of the present technology. Such space segment forward link equipment
300, which can also be referred to as a forward link satellite subsystem 300, or more
generally, as an optical communication subsystem, is configured to receive the optical
signal that is transmitted from the ground based optical gateway subsystem 200 to
the satellite that is carrying the space segment forward link equipment 300. The space
segment forward link equipment 300 is also configured to convert the optical signal
that it receives (from the ground based optical gateway subsystem 200) into electrical
signals, and to produce service beams therefrom, wherein the service beams are for
transmission from the satellite to service terminals ST.
[0038] Referring to Figure 3, the forward link satellite subsystem 300 is shown as including
receiver optics 302, an optical amplifier (OA) 304, a wavelength-division multiplexing
(WDM) demultiplexer (DEMUX) 306, two hundred and fifty photodetectors (PDs) 308_1
to 308_250, two hundred and fifty filters 310_1 to 310_250, and two hundred and fifty
low noise amplifiers (LNAs) 312_1 to 312_250. The forward link satellite subsystem
300 is also shown as including two hundred and fifty filters (FTRs) 316_1 to 316_250,
high power amplifiers (HPAs) 318_1 to 318_250, harmonic filters (HFs) 320_1 to 320_250,
test couplers (TCs) 322_1 to 322_250, orthomode junctions (OMJs) 324_1 to 324_250,
and feed elements 326_1 to 326_250. The PDs 308_1 to 308_250 can be referred to individually
as a PD 308, or collectively as the PDs 308. The filters 310_1 to 310_250 can be referred
to individually as a filter 310, or collectively as the filters 310. The LNAs 312_1
to 312_250 can be referred to individually as an LNA 312, or collectively as the LNAs
312. The filters 316_1 to 316_250 can be referred to individually as a filter 316,
or collectively as the filters 316. The HPAs 318_1 to 318_250 can be referred to individually
as an HPA 318, or collectively as the HPAs 318. The HFs 320_1 to 320_250 can be referred
to individually as an HF 320, or collectively as the HFs 320. The TCs 322_1 to 322_250
can be referred to individually as a TC 322, or collectively as the TCs 322. The OMJs
324_1 to 324_250 can be referred to individually as an OMJ 324, or collectively as
the OMJs 324. The feed elements 326_1 to 326_250 can be referred to individually as
a feed element 326, or collectively as the feed elements 326.
[0039] The receiver optics 302, which can also be referred to as a telescope, can includes
optical elements such as mirrors, reflectors, filters and/or the like. The receiver
optics 302 receives the optical feeder uplink beam that is transmitted through free-space
to the satellite by the ground based optical gateway forward link subsystem 200, and
provides the received optical feeder uplink beam (e.g., via an optical fiber) to the
OA 304. A gimbal, and/or the like, can be used to control the steering of the receiver
optics 302. When the optical feeder uplink beam reaches the satellite, the power of
the optical feeder uplink beam is significantly attenuated compared to when it was
transmitted by the ground based optical gateway subsystem 200. Accordingly, the OA
304 is used to amplify the received optical feeder uplink beam before it is provided
to the WDM DEMUX 306. The OA 304 can be, e.g., an erbium-doped fiber amplifier (EDFA),
but is not limited thereto. The output of the OA 304 can be referred to as an optically
amplified received optical feeder uplink beam. The WDM DEMUX 306 demultiplexes (i.e.,
separates) the received optical feeder uplink beam (after it has been optically amplified)
into two hundred and fifty separate optical signals, each of which is provided to
a separate photodetector (PD) 308. Each PD 308 converts the optical signal it receives
from the WDM DEMUX 306 to a respective RF electrical signal. The RF electrical signal
produced by each PD 308 is provided to a respective filter (FTR) 310 (e.g., a bandpass
filter) to remove unwanted frequency components and/or enhance desired frequency components.
For an example, each filter 310 can pass frequencies within the range of 17.7 - 20.2
GHz, or within the range of 17.3 - 20.2 GHz, but are not limited thereto. The filtered
RF electrical signal, which is output by each filter 310, is provided to a respective
low noise amplifier (LNA) 312. Each LNA 312 amplifies the relatively low-power RF
signal it receives from a respective filter 310 without significantly degrading the
signals signal-to-noise ratio. The amplified RF signal that is output by each LNA
312 is provided to a filter 316. The filter 316_1 may have a passband of 17.7 - 20.2
GHz, or 17.3 - 20.2 GHz, but are not limited thereto.
[0040] Each HPA 318 amplifies the RF signal it receives so that the RF signal has sufficient
power to enable transmission thereof from the satellite 100 in space to a service
terminal ST, which may be on the ground. Each HPA 318 can be, e.g., a linearized traveling
wave tube high power amplifier, but is not limited thereto. The signal that is output
by each of the HPAs 318 can be referred to as an amplified RF signal. Each HF 320
is used to reduce out-of-band emissions caused by the nonlinearity caused by a respective
HPA 318. Each HF 320 can be, e.g., a waveguide cavity filter, but is not limited thereto.
Each test coupler TC 322 can be used for power monitoring, payload testing and/or
performing calibrations based on signals passing therethrough. Each OMJ 324 adds either
right hand circular polarization (RHCP) or left hand circular polarization (LHCP)
to the RF signal that is passed through the OMJ. This allows for color reuse frequency
band allocation, wherein each color represents a unique combination of a frequency
band and an antenna polarization. This way a pair of feeder beams that illuminate
adjacent regions can utilize a same RF frequency band, so long as they have orthogonal
polarizations. Alternatively, each OMJ 324 adds either horizontal linear polarization
or vertical linear polarization to the RF signal that is passed through the OMJ. Each
feed element 326, which is an example of a feed element, converts the RF signal it
receives, from a respective OMJ 324, to radio waves and feeds them to the rest of
the antenna system (not shown) to focus the signal into a service downlink beam. A
feed element326 and the rest of an antenna can be collectively referred to as the
antenna subsystem. All or some of the feed elements 326 can share a common reflector.
Such reflector(s) is/are not shown in the Figures, to simply the Figures. The two
hundred and fifty feed elements , element326_1 to 326_250, form a multiple element
antenna feed array. This multiple element antenna feed array is used to form spot
beams (e.g., one thousand spot beams) as controlled by the GBBF 230.
Space Segment Return Link Equipment
[0041] Figure 4A will now be used to describe a portion of space segment return link equipment
400A, according to an embodiment of the present technology. Such space segment return
link equipment 400A, which can also be referred to as a satellite return link subsystem
400A, or more generally, as an optical communication subsystem, is configured to receive
the RF signals that are transmitted by service terminals ST to the satellite (e.g.,
100) that is carrying the space segment return link equipment 400A. The space segment
return link equipment 400A, together with the space segment return link equipment
400B in Figure 4B, is also configured to convert the RF signals that it receives (from
the service terminals ST) into optical signals, and to produce optical return feeder
beams therefrom, wherein the optical return feeder beams are for transmission from
the satellite (e.g., 100) to a ground based gateway (e.g., 105).
[0042] Referring to Figure 4A, the portion of the space segment return link equipment 400A
shown therein includes feed elements 402_1 to 402_250 (which can be referred to individually
as a feed element 402, or collectively as the feed elements 402), orthomode junctions
(OMJs) 404_1 to 404_250 (which can be referred to individually as an OMJ 404, or collectively
as the OMJs 404), test couplers (TCs) 406_1 to 406_250 (which can be referred to individually
as a TC 406, or collectively as the TCs 406), pre-select filters (PFs) 408_1 to 408_250
(which can be referred to individually as a PF 408, or collectively as the PFs 408),
low noise amplifiers (LNAs) 410_1 to 410_250 (which can be referred to individually
as an LNA 410, or collectively as the LNAs 410), and filters (FTRs) 412_1 to 412_250
(which can be referred to individually as a filter 412, or collectively as the filters
412). The portion of the space segment return link equipment 400A shown in Figure
4A also includes frequency down-converters (FDCs) 416_1 to 416_250 (which can be referred
to individually as a frequency down-converter 416, or collectively as the frequency
down-converters 416), filters (FTRs) 418_1 to 418_250 (which can be referred to individually
as a filter 418, or collectively as the filters 418), and local oscillators (LOs)
422_1 to 422_10 (which can be referred to individually as an LO 422, or collectively
as the LOs 422). The portion of the space segment return link equipment 400A shown
in Figure 4A also includes combiners 420_1 to 420_25 (which can be referred to individually
as a combiner 420, or collectively as the combiners 420).
[0043] Each feed element 402 gathers and focuses radio waves of a service uplink beam (e.g.,
106u) and converts them to an RF signal that is provided to a respective OMJ 404.
A feed element 402 and the rest of an antenna can be collectively referred to as the
antenna or antenna system. In other words, an antenna, as the term is used herein,
can include a feed element. All or some of the feed elements 402 can share a common
reflector. Such reflector(s) is/are not shown in the Figures, to simply the Figures.
Each OMJ 404 either passes through a right hand circular polarization (RHCP) or a
left hand circular polarization (LHCP) RF signal. Each OMJ 404 can alternatively pass
through either a horizontal or a vertical linear polarization RF signal. Each test
coupler TC 406 can be used for power monitoring, payload testing and/or performing
calibrations based on signals passing therethrough. Each pre-select filter (PF) 408
(e.g., a bandpass filter) is used to remove unwanted frequency components and/or enhance
desired frequency components. For an example, each PF 408 can pass frequencies within
the range of 29.5 - 30.0 GHz, but is not limited thereto. Each LNA 410 amplifies the
relatively low-power RF signal it receives from a respective PF 408 without significantly
degrading the signals signal-to-noise ratio. The amplified RF signal that is output
by each LNA 410 is provided to a respective filter 412.
[0044] Each filter 412 allows frequencies to pass within a specified frequency range (e.g.,
29.50 - 30.00 GHz), and the filters 418 that are downstream of the frequency down-converters
416 are used to filter out unwanted frequency components (e.g., unwanted mixed products)
that result from the frequency down-conversions.
[0045] Each frequency down-converter 416 receives an RF signal from a filter 412 (which
RF signal includes data from a uplink beam, and thus, can be referred to as an RF
data signal) and an RF signal from an LO 422 (which can be referred to as an LO signal),
and uses the LO signal to down-convert the RF data signal to a frequency range (e.g.,
6.70 - 7.2 GHz, or 6.3 - 7.2 GHz, or some other frequency range within the 6 - 12
GHz band) that can be used for transmitting feeder downlink signals (e.g., 102d) to
a gateway (e.g., 105). The output of each frequency down-converter 416 is provided
to a filter 418. For example, the frequency down-converter 416_1 provides its output
to the filter 418_1, and the frequency down-converter 416_2 provides its output to
the filter 418_2. The filter 418_1 can be a bandpass filter that allows frequencies
to pass within a specified frequency range (e.g., 6.70 - 7.2 GHz, or 6.3 - 7.2 GHz,
or some other frequency range within the 6 - 12 GHz band).
[0046] In the embodiment of Figure 4A, the outputs of ten filters 418 are provided to a
combiner 420. For example, the outputs of filters 418_1, 418_2, 418_3 ... 418_10 are
provided the combiner 420_1. Each combiner 420 combines the ten down-converted and
filtered signals it receives into a combined signal that includes data modulated RF
carriers for ten service uplink beams. In other words, the output of each combiner
420 includes data received from ten service uplink beams associated with at least
ten service terminals ST. The output of each combiner 420 is provided to a separate
EOM 434, as will be discussed below with reference to Figure 4B.
[0047] Figure 4B will now be used to describe a further portion of the space segment return
link equipment 400B that is used to convert the data modulated RF carrier signals
into a collimated optical downlink feeder beam that is aimed at a gateway. Referring
to Figure 4B, the portion of the space segment return link equipment 400B is shown
as including twenty five lasers 432_1 to 432_25, twenty five electro-optical modulator
(EOMs) 434_1 to 434_25, a wavelength-division multiplexing (WDM) multiplexer (MUX)
436, an optical amplifier (OA) 438 and transmitter optics 440. Each of these elements
are described below.
[0048] The twenty five separate lasers 432_1 to 432_25 each emit light of a different wavelength
within a specified wavelength range. The lasers can be referred to individually as
a laser 432, or collectively as the lasers 432. Where the specified wavelength range
is, for example, from 1560 nm to 1570 nm, then the laser 432_1 may emit light having
a peak wavelength of 1560 nm, the laser 432_2 may emit light having a peak wavelength
of 1560.4 nm, the laser 432_3 (not shown) may emit light having a peak wavelength
of 1560.8 nm, ... the laser 432_24 may emit light having a peak wavelength of 1669.6
nm, and the laser 432_25 may emit light having a peak wavelength of 1670.0 nm. In
other words, the peak wavelengths emitted by the lasers 432 can occur at 0.4 nm intervals
from 1560 nm to 1570 nm. The wavelength range from 1560 nm to 1570 nm, which is within
the IR spectrum, is practical to use because IR lasers for use in communications are
readily available. However, wider or narrow wavelength ranges, within the same or
other parts of the optical spectrum, may alternatively be used. For example, it would
also be possible to utilize a wavelength range within the 400 nm - 700 nm visible
spectrum. It is also possible that the wavelength range that is specified for use
in producing the optical feeder downlink beam (e.g., 102d) is non-contiguous. For
example, the wavelength range that is for use in producing the optical feeder downlink
beam can be from 1535 nm to 1540 nm and from 1565 nm to 1575 nm. These are just a
few examples, which are not intended to be all encompassing. Further, it is also possible
that space segment return link equipment can alternatively include more or less than
twenty five lasers (that each emit light of a different peak wavelength within a specified
contiguous or non-contiguous wavelength range). Additionally, it is noted that the
space segment return link equipment may include two or more of each of the lasers
(that each emit light of a different peak wavelength within a specified contiguous
or non-contiguous wavelength range) to provide for redundancy or backup. Each of the
lasers 432 can be, for example, a diode-pumped infrared neodymium laser, although
the use of other types of lasers are also within the scope of the embodiments described
herein.
[0049] In accordance with certain embodiments, the space segment return link equipment 400B
includes less lasers (e.g., twenty five lasers 432) for use in generating the optical
feeder downlink beam that is aimed from the satellite 100 to the gateway 105, than
the gateway forward link equipment 200 includes (e.g., two hundred and twenty five
lasers 202) for generating the optical feeder uplink beam that is aimed from the gateway
105 to the satellite 100. This is made possible due to current asymmetric capacity
requirements between the forward and return feeder links. More specifically, a feeder
downlink beam (e.g., 102d) carries significantly less data than a feeder uplink beam
(e.g., 102u), because service terminals ST typically download much more data than
they upload.
[0050] On the return link, given the current asymmetric capacity requirements between the
forward and return links, the space segment return link equipment can be implemented
to handle less demand that the ground based forward link equipment. As an example,
if each RF service uplink beam is assumed to have only 320 MHz per beam, then a total
of 160 GHz needs to be sent from a satellite to a gateway on the optical feeder downlink
beam. Several beams' frequencies can be grouped together to create a 4 GHz bandwidth
which is then transmitted on each of twenty five laser wavelengths that are multiplexed
together and transmitted to the ground. An alternative implementation would be to
aggregate the 4 GHz spectrum with filtering post LNA to eliminate the RF frequency
conversion and as above directly modulate the RF spectrum on each of the twenty five
laser wavelengths. An alternative implementation would be to use only RF LNAs for
each feed, modulate each 320 MHz segment of bandwidth onto a single laser and combine
two hundred and twenty five laser wavelengths together, thus eliminating the need
for RF frequency converters. Depending on the number of service beams and feeder beams
required, one or the other configuration can be selected to provide the lowest mass
solution.
[0051] The light emitted by each of the twenty five lasers 432, which can be referred to
as an optical carrier signal, is provided (e.g., via a respective optical fiber) to
a respective one of the twenty five separate EOMs 434_1 to 434_25. The EOMs can be
referred to individually as an EOM 434, or collectively as the EOMs 434. Each of the
EOMs 434 is an optical device in which a signal-controlled element exhibiting an electro-optic
effect is used to modulate a respective beam of light. The modulation performed by
the EOMs 434 may be imposed on the phase, frequency, amplitude, or polarization of
a beam of light, or any combination thereof. In accordance with a specific embodiment,
each of the EOMs 434 is a phase modulating EOM that is used as an amplitude modulator
by using a Mach-Zehnder interferometer. In other words, each of the EOMs 434 can be
implemented as a Mach-Zehnder modulator (MZM), which can be a Lithium Niobate Mach-Zehnder
modulator, but is not limited thereto. In accordance with specific embodiments, each
of the EOMs 434 is implemented as an MZM that produces an amplitude modulated (AM)
optical waveform with a modulation index between 10% and 80% in order to maintain
fidelity of an RF waveform (modulated therein) without too much distortion. The optical
signal that is output by each of the EOMs 434 can be referred to as an optical data
signal. The modulation scheme that is implemented by the EOMs 434 can result in double-
or vestigial-sidebands, including both an upper sideband (USB) and a lower sideband
(LSB). Alternatively single-sideband modulation (SSB) can be utilized to increase
bandwidth and transmission power efficiency.
[0052] The twenty five separate optical data signals that are output by the fifty EOMs 434
are provided to the WDM MUX 436, which can also be referred to as a dense wavelength
division multiplexing (DWDM) MUX. The WMD MUX 436 multiplexes (i.e., combines) the
twenty five optical data signals, received from the twenty five EOMs 434, onto a single
optical fiber, with each of the twenty five separate optical data signals being carried
at the same time on its own separate optical wavelength within a specified contiguous
wavelength range (e.g., from 1560 nm to 1570 nm) or non-contiguous wavelength range
(e.g., from 1510 nm to 1535 nm, and from 1540 nm to 1565 nm). For example, as explained
above, the twenty five optical data signals can have peak wavelengths that occur at
0.4 nm intervals from 1560 nm to 1570 nm, but are not limited thereto.
[0053] The signal that is output by the WMD MUX 436, which can be referred to as a wavelength
division multiplexed optical signal, is provided to the optical amplifier (OA) 438.
The OA 438 amplifies the wavelength division multiplexed optical signal so that the
wavelength division multiplexed optical signal has sufficient power to enable transmission
thereof from the satellite 100 in free-space to the gateway 105. The OA 438 can be
an erbium-doped fiber amplifier (EDFA), but is not limited thereto. The output of
the OA 438 can be referred to as an optically amplified wavelength division multiplexed
optical signal.
[0054] The optically amplified wavelength division multiplexed optical signal, which is
output by the OA 438, is provided (e.g., via an optical fiber) to the transmitter
optics 440. The transmitter optics 440, which can also be referred to as a telescope,
can includes optical elements such as lenses, mirrors, reflectors, filters and/or
the like. The transmitter optics 440 outputs a collimated optical feeder downlink
beam that is aimed at a satellite. A gimbal, and/or the like, can be used to control
the steering of the transmitter optics 440. In accordance with an embodiment, the
collimated optical feeder downlink beam has an aperture of about 40 cm, and a half
beam divergence of about 0.0000012 radians, wherein the term "about" as used herein
means +/- 10 percent of a specified value. The use of other apertures and half beam
divergence values are also within the scope of the embodiments described herein. The
collimated optical feeder downlink beam, which is output by the transmitter optics
440, is transmitted in free-space to receiver optics in the gateway 105.
[0055] A space segment (e.g., a satellite 100) can have different optics that are used for
transmitting an optical feeder downlink beam (e.g., 102d) to a gateway, than the optics
that are used for receiving an optical feeder uplink beam (e.g., 102u) from a gateway.
Alternatively, and preferably, to reduce the weight that needs to be carried by the
space segment (e.g., a satellite 100), the same optics can be used for both transmitting
an optical feeder downlink beam (e.g., 102d) to a gateway and for receiving an optical
feeder uplink beam (e.g., 102u) from a gateway. More specifically, the TX optics 440
shown in Figure 4B can be the same as the RX optics 302 shown in Figure 3. Additional
and/or alternative components can be shared between the space segment forward link
equipment shown in Figure 3 and the space segment return link equipment shown in Figures
4A and 4B. For example, the feed elements 326 in Figure 3 can be the same as the feed
elements 402 shown in Figure 4A. For another example, the OMJs 324 in Figure 3 can
be the same as the OMJs 404 in Figure 4A, if the OMJs are implement as a three-port
device. These are just a few example, which are not intended to be all encompassing.
[0056] Referring again to the EOMs 434 in Figure 4B, in accordance with certain embodiments
of the present technology, each of the EOMs 434 modulates the optical signal it receives
(e.g., via an optical fiber from a respective laser 432) with a separate RF signal
that has already been modulated to include user data. For example, the EOM 434_1 modulates
the optical signal it receives from the laser 431_1 with a data modulated RF carrier
signal it receives from the combiner 420_1 (in Figure 4A). The data modulated RF carrier
signal that the EOM 434_1 receives from a combiner (420_1 in Figure 4A) can include
data corresponding to ten service uplink beams received from service terminals ST.
Similarly, the EOMs 434_2 to 434_50 can each receive a different data modulated RF
carrier signal, from a different combiner 420, with each data modulated RF carrier
signal corresponding to a different group of ten service uplink beams received from
service terminals ST. In this manner, the EOMs 434 can be collectively provided with
data modulated RF carrier signals corresponding to two hundred and fifty service uplink
beams (i.e., 25 * 10 = 250).
Gateway Return Link Equipment
[0057] Figure 5A will now be used to describe gateway return link equipment 500, according
to an embodiment of the present technology. Such gateway return link equipment 500
can also be referred to as an optical gateway return link subsystem 500, or more generally,
as an optical communication subsystem. Referring to Figure 5A, the optical gateway
return link subsystem 500 is shown as including receiver optics 502, an optical amplifier
(OA) 504, a wavelength-division multiplexing (WDM) demultiplexer (DEMUX) 506, twenty
five photodetectors (PDs) 508_1 to 508_25, twenty five filters (FTRs) 510_1 to 510_25,
twenty five low noise amplifiers (LNAs) 512_1 to 512_25, and twenty five frequency
down-converters (FDCs) 514_1 to 514_25. The optical gateway return link subsystem
500 is also shown as including a ground based beamformer (GBBF) 515, M demodulator
and digital signal processor (DSP) blocks 516_1 to 516_M, and twenty five local oscillators
(LOs) 522_1 to 522_25 (which can be referred to individually as an LO 522, or collectively
as the LOs 522).
[0058] The receiver optics 502, which can also be referred to as a telescope, can includes
optical elements such as mirrors, reflectors, filters and/or the like. The receiver
optics 502 receives the optical feeder downlink beam (e.g., 102d) that is transmitted
through free-space from a space segment (e.g., a satellite 100), by the space based
return link subsystem 400A and 400B, and provides the received optical feeder downlink
beam (e.g., via an optical fiber) to the OA 504. A gimbal, and/or the like, can be
used to control the steering of the receiver optics 502. When the optical feeder downlink
beam reaches the gateway, the power of the optical feeder downlink beam is significantly
attenuated compared to when it was transmitted by the space based return link subsystem.
Accordingly, the OA 504 is used to amplify the received optical feeder downlink beam
before it is provided to the WDM DEMUX 506. The OA 504 can be, e.g., an erbium-doped
fiber amplifier (EDFA), but is not limited thereto. The output of the OA 504 can be
referred to as an optically amplified received optical feeder downlink beam. The WDM
DEMUX 506 demultiplexes (i.e., separates) the received optical feeder uplink beam
(after it has been optically amplified) into fifty separate optical signals, each
of which is provided to a separate photodetector (PD) 508. Each PD 508 converts the
optical signal it receives from the WDM DEMUX 506 to a respective RF electrical signal.
The RF electrical signal produced by each PD 508 is provided to a respective filter
(FTR) 510 (e.g., a bandpass filter) to remove unwanted frequency components and/or
enhance desired frequency components. For an example, where frequency down-conversions
were performed on the satellite (by the FDCs 416 of the space segment return link
equipment 400A), each filter 510 can pass frequencies within the range of 6.70 - 7.2
GHz, or within the range of 6.3 - 7.2 GHz, but are not limited thereto. For another
example, where frequency down-conversions were not performed on the satellite, each
filter 510 can pass frequencies within the range of 29.5 - 30 GHz, but are not limited
thereto. The filtered RF electrical signal, which is output by each filter 408, is
provided to a respective low noise amplifier (LNA) 512. Each LNA 512 amplifies the
relatively low-power RF signal it receives from a respective filter 510 without significantly
degrading the signals signal-to-noise ratio. The amplified RF signal that is output
by each LNA 512 is provided to a respective frequency down-converter 514, the output
of which is provided to the GBBF 515, which outputs signals provided to demodulator
and DSP blocks 516.
[0059] Each frequency down-converter 514 receives an RF signal from an LNA 512 (which RF
signal includes data from subscriber terminals STs, and thus, can be referred to as
an RF data signal) and an RF signal from an LO 452 (which can be referred to as an
LO signal), and uses the LO signal to down-convert the RF data signal to baseband.
The baseband data signal output by each frequency down-converter 514 is provided to
the GBBF 515. The GBBF 515 can be referred to more specifically as the return link
GBBF 515, so as to distinguish it from the GBBF 230, which can be referred to more
specifically as the forward link GBBF 230. The return link GBBF 515 uses calibration
information received from a calibration subsystem, to produce M baseband spot beams
(where, M can equal 1000 beams as on the forward link, or in some systems, the forward
and return beam counts could be different from one another) Each of the M spot beam
signals is provided to a respective demodulator and DSP block 516. Each demodulator
and DSP block 516 demodulates the baseband spot beam signal it receives, and performs
digital signal processing thereon. Such a demodulated data signal can be used to provide
data to, or request data from, a server, client and/or the like that is coupled to
a network (e.g., the network 140 in Figure 1).
[0060] A gateway (e.g., 105) can have different optics that are used for transmitting an
optical feeder uplink beam (e.g., 102u) to a space segment (e.g., satellite 100),
than the optics that are used for receiving an optical feeder downlink beam (e.g.,
102d) from a space segment. Alternatively, a gateway can use the same optics for both
transmitting an optical feeder uplink beam (e.g., 102u) to a space segment and for
receiving an optical feeder downlink beam (e.g., 102d) from a space segment. More
specifically, the RX optics 502 shown in Figure 5A can be the same as the TX optics
210 shown in Figure 2A.
[0061] Figure 5B will now be used to provide details of the return link GBBF 515 introduced
in Figure 5A, according to certain embodiments of the present technology. Referring
to Figure 5B, the GBBF 515 is shown as including a GBBF controller 532 and twenty
five one-to-ten (1 to 10) splitters 534_1 to 534_25. Each individual splitter 534_1
to 534_25 (which can be referred to collectively as the splitters 534, or individually
as a splitter 534) receives a respective one of the twenty five aggregate signals
(aggregate signal_1, aggregate signal_2 ... aggregate signal_25) and outputs ten copies
of the aggregate signal received by the splitter 534. The aggregate signal received
by each of the splitters 524 is an aggregate of ten feed element signals, each of
which corresponds to an output of one of the combiners 420 shown in and described
above with reference to Figure 4A (which output is provided to one of the EOMs 434
in Figure 4B). The ten outputs of each of the splitters 534 are provided to a different
one of ten frequency down converters (FDCs) 536. Each of the FDCs receives a different
RF signal from a different one of ten local oscillators (LOs) 516_1 to 516_10 (which
can be referred to individually as an LO 516, or collective as the LOs 516), and thereby
frequency shifts each of the copies of an aggregate signal. This way when the ten
separately frequency shifted copies of the aggregate signal are passed through a respective
one of the filters (FTRs) 538, which have a common band pass frequency range, the
ten feed elements signals that were included in an aggregate signal are separated
into ten separate feed element signals at the outputs of the FTRs 538. In the embodiment
shown in Figure 5B, there are twenty five groups of ten FDCs 536, and thus a total
of two hundred and fifty FDCs 536, and there are a total of twenty five groups of
ten FTRs 538, and thus a total of two hundred and fifty FTRs 538. In the manner described
above, the outputs of the two hundred and fifty filters 538 are two hundred and fifty
feed element signals that correspond to the outputs of the FTRs 418 described above
with reference to Figure 4A. In accordance with certain embodiments, the functions
of the splitters 534, the FDCs 536, the FTRs 538 and the splitters 540 are performed
using a digital signal processor (DSP) instead of N separate splitters. In other words,
a DSP can perform the signal copying or replication.
[0062] In order to form one thousand spot beams signals from the two hundred and fifty feed
element signals (that are respectively output from the two hundred and fifty FTRs
538 in Figure 5B), each of the feed element signals is provided to a respective one-to-one
thousand (1 to 1000) splitter 540. Each individual splitter 540 receives a respective
one of the the two hundred and fifty feed element signals and outputs one thousand
copies of the feed element signal received by the splitter 540. Each copy of a feed
element signals is provided to a respective phase and amplitude weight elements 542,
which can be implemented in hardware, but are most likely implemented in software
and/or firmware. The GBBF controller 532 uses calibration signals (received from a
calibration subsystem) to derive element specific amplitude and phase corrections
that are provided to the phase and amplitude weight elements 542, to thereby apply
the corrections to the component element signals provided thereto. An output of each
of the phase and amplitude weight elements 542 is provided to one of one thousand
summers 544_1 to 544_1000 (which can be referred to individually as a summer 544,
or collectively as the summers 544). The outputs of the summers 544_1 to 544_1000
are the baseband spot beam signals that are provided to a spot beam to user data controller
550, which can be considered part of the GBBF 515, or can be considered external thereto.
In the embodiment shown, the spot beam to user data controller 550 maps the one thousand
spot beam signals output by the summers 544 to ten thousand user data signals. The
user data signals are what can be provided to the demodulator and DSP blocks 516 shown
in Figure 5A, which was discussed above.
[0063] The GBBF 515 can be implemented entirely in software. Alternatively, or additionally,
the GBBF 515 can be implemented in hardware and/or firmware. The outputs of the phase
and amplitude weight elements 542 can be referred to a component element signals.
The outputs of the summers 544, which outputs are referred to as the baseband spot
beam signals above, can also be referred to as composite signals, since they are a
composite of a plurality of component element signals. The splitters 534 and 540 and
the weighting elements 542 can be collectively referred to as a signal replication
and reverse beamforming weighting unit 533. The same calibration subsystem that provides
calibration signals to the GBBF controller 232 can provide calibration signals to
the GBBF controller 532.The GBBF controller 532, which receives the calibration signals,
can be the same as the GBBF controller 232, or distinct therefrom.
[0064] As noted above, a DSP can perform the signal copying or replication performed by
the splitters 534, the FDCs 536, the FTRs 538 and the splitters 540. More generally,
all of the functions of the elements described within the block labeled GBBF 515 can
be implemented by a DSP. In other words, the GBBF 515 can be entirely or substantially
entirely implemented using a DSP. Nevertheless, it is useful to shown and describe
the elements shown in FIG. 5B in order to understand the operation of such a DSP.
The spot beam to user data controller 550 can also be implemented by the DSP.
[0065] Embodiments of the present technology described herein enable analog over free space
optics (AoFSO) technology to be used on gateway to/from satellite links, replacing
the normal Ku or Ka or V band spectrum. As described above, analog modulation of a
number of wavelengths of light that are Wavelength Division Multiplexed (WDM) into
a single laser beam from earth to space, and detected by photodetectors on the satellite.
By modulating these optical wavelengths at the desired RF frequencies, it is possible
to use GBBF, even at high frequencies like Ka band, and with large numbers of feed
elements, due to the high RF bandwidth available at optical frequencies.
[0066] Currently envisioned free space optical spacecraft architectures use demodulation
of the optical signal, followed by routing to user link pathways and remodulation
of the signal on user link RF frequencies. Using embodiments of the present technology
described herein it is also possible for ground based beamforming to be used within
a gateway where a channelizer and/or router onboard a satellite may or may not be
present, but the input segments from the feederlink on the forward link (or to the
feederlink on the return link) are routed to (onboard) feed elements meant to form
user beams (also referred to as spot beams). In embodiments of the present technology
that use ground based beamforming, the ground system creates and uses the proper amplitude
and phase coefficients to form the desired user beams (also referred to as spot beams).
on a dynamic basis. By contrast, in an onboard beamforming (OBBF) system, the phase
and amplitude coefficients are typically created onboard a satellite using analog
or digital hardware. In any case, there is a one-to-one correspondence between the
number of feed elements onboard and the number of feederlink band segments, also referred
to as feed element signals, carrying signals for that feed. With a large number of
feed elements and a high bandwidth per beam, it becomes impossible to find the feederlink
spectrum needed at RF frequencies, but is not a problem at optical frequencies.
[0067] An advantage of the optical approach for HTS broadcast and other satellite applications
is that it allows for flexible antenna beam forming with GBBF for large signal bandwidth
without the limitation associated with the available gateway uplink and downlink spectrum
at RF frequencies. This approach allows for the flexible generation of multiple beams
from an array fed reflector. In addition, in embodiments where the communication signal
is modulated at the transmit (forward) and receive (return) RF frequencies when applicable
no frequency conversion is required on the satellite, further simplifying the payload
design. However for increased flexibility, frequency conversion can still be applied
on the satellite if desired.
Methods
[0068] Figure 6 will now be used to summarize methods for enabling a ground based subsystem
(e.g., the gateway forward link equipment 200 in Figure 2A) to produce and transmit
an optical feeder uplink beam (e.g., 102u in Figure 1) to a satellite (e.g., 100 in
Figure 1) that includes a multiple element antenna feed array and that is configured
to receive the optical feeder uplink beam and in dependence thereon use the multiple
element antenna feed array to produce and transmit a plurality of RF service downlink
beams (e.g., 106d, 110d, 114d and 118d in Figure 1) to service terminals ST. In accordance
with certain embodiments, a specified RF frequency range within which the satellite
is configured to produce and transmit a plurality of RF service downlink beams is
a downlink portion of the Ka band. The downlink portion of the Ka band can be from
17.7 GHz to 20.2 GHz, and thus, have a bandwidth of 2.5 GHz. Alternatively, the downlink
portion of the Ka band can be from 17.3 GHz to 20.2 GHz, and thus, have a bandwidth
of 2.9 GHz. These are just a few examples, which are not intended to be all encompassing.
[0069] Referring to Figure 6, step 602 involves performing ground based beamforming by receiving
a plurality of spot beam signals, producing or otherwise obtaining phase and amplitude
beamforming coefficients, and producing a plurality of feed element signals in dependence
on the plurality of spot beam signals and the phase and amplitude beamforming coefficients.
Step 602 can be performed, e.g., by the GBBF 230 described above with reference to
Figures 2A and 2B. Prior to step 602, the method can involve receiving a set of user
data signals (e.g., ten thousand user data signals), and combining subsets of the
user data signals into the spot beam signals (e.g., one thousand spot beam signals)
that are used for the ground based beamforming, which steps can be performed by the
user data to spot beam controller 220 discussed above with reference to Figure 2A.
[0070] Still referring to Figure 6, step 604 involves emitting a plurality of optical signals
each having a different peak wavelength that is within a specified optical wavelength
range. Step 604 can be performed by the lasers 202 described above with reference
to Figure 2A. The specified optical wavelength range may be within the C-band and/or
L-band optical wavelengths, as explained above. Further, as explained above, the specified
optical wavelength range can be a contiguous optical wavelength range within an IR
spectrum, or a non-contiguous optical wavelength range within the IR spectrum. As
noted above, visible and/or other optical wavelengths may alternatively be used.
[0071] Step 606 involves electro-optically modulating each of the optical carrier signals
with one of the feed element signals produced by the ground based beamforming to thereby
produce a plurality of optical feed element signals. Step 606 can be performed by
the EOMs 204 described above with reference to Figure 2A. The feed element signals
may be frequency up-converted (e.g., by the FUCs 244 in Figure 2A) and filtered (e.g.,
by the FTRs 246 in Figure 2A) prior to the electro-optically modulating
[0072] Step 608 involves multiplexing the plurality of optical data signals to thereby produce
a wavelength division multiplexed optical signal that includes data for the plurality
of RF service downlink beams. Step 610 can be performed using the WDM MUX 206 discussed
above with reference to Figure 2A.
[0073] Step 610 involves producing an optical feeder uplink beam, in dependence on the wavelength
division multiplexed optical signal, and step 612 involves transmitting the optical
feeder uplink beam through free-space to the satellite. Steps 610 and 612 can be performed
by the transmitter optics 210 discussed above with reference to Figure 2A. The optical
amplifier (OA) 208 discussed above with reference to Figure 2A can also be used to
perform step 610.
[0074] In accordance with certain embodiments, each of the plurality of optical data signals
resulting from the electro-optically modulating at step 608 has an RF frequency within
the same specified RF frequency range within which the satellite (e.g., 100) is configured
to transmit the plurality of RF service downlink beams. In such embodiments, beneficially,
because RF frequencies of the optical data signals resulting from the electro-optically
modulating are within the same specified RF frequency range within which the satellite
is configured to transmit the plurality of RF service downlink beams, there is an
elimination of any need for the satellite to perform any frequency conversions when
producing the plurality of RF service downlink beams in dependence on the optical
feeder uplink beam. In other words, the space segment forward link equipment 300 in
Figure 3 beneficially does not need any frequency down-converters or any other type
of frequency conversion equipment.
[0075] Additional details of step 602 according to certain embodiments of the present technology,
which can be performed by the GBBF 230 described above with reference to Figures 2A
and 2B, will now be described with reference to Figure 7. More specifically, Figure
7 is used to explain additional details of how the ground based beamforming can be
performed at step 602. Referring to Figure 7, step 702 involves produces multiple
copies of each of the spot beam signals. Step 702 can be performed by the splitters
234 described above with reference to Figure 2B.
[0076] Still referring to Figure 7, step 704 involves using the phase and amplitude coefficients
to weight different copies of the spot beam signals in different manners. Step 704
can be performed by the weight elements 236 described above with reference to Figure
2B.
[0077] Step 706 involves summing subsets of the weighted copies of the spot beam signals
to thereby produced the feed element signals. Step 706 can be performed by the summers
238 described above with reference to Figure 2B. The feed element signals produced
at step 706 may be frequency up-converted and filtered prior to the electro-optically
modulating that is performed at step 604.
[0078] More generally, steps 702, 704 and 706 can be performed by the signal replication
and forward beamforming weighting unit 233 described above with reference to Figure
2B.
[0079] Further details of the methods described with reference to Figures 6 and 7 can be
appreciated from the above description of Figures 1-5.
[0080] Certain embodiments of the present technology described above relate to a ground
based subsystem for use in transmitting an optical feeder uplink beam to a satellite
that includes a multiple element antenna feed array and that is configured to receive
and/or input the optical feeder uplink beam and in dependence thereon use the multiple
element antenna feed array to produce and transmit a plurality of RF service downlink
beams to service terminals. The ground based subsystem can include a ground based
beamformer (GBBF), a plurality of lasers, a plurality of electro-optical modulators
(EOMs), a wavelength-division multiplexing (WDM), an optical amplifier, and transmitter
optics. The GBBF can be configured to accept a plurality of spot beam signals, produce
or otherwise obtain phase and amplitude beamforming coefficients, and output a plurality
of feed element signals in dependence on the plurality of spot beam signals and the
phase and amplitude beamforming coefficients. Each of the lasers can be operable to
emit an optical signal having a different peak wavelength within a specified optical
wavelength range. Each EOM of the plurality of EOMs can be configured to accept an
optical carrier signal from a respective one of the plurality of lasers, accept a
different one of the plurality of feed element signals from the GBBF, and output a
respective optical feed element signal in dependence on the optical carrier signal
and the feed element signal accepted by the EOM. The WDM multiplexer can be configured
to accept the optical feed element signals output by the plurality of EOMs, and combine
the plurality of optical feed element signals into a wavelength division multiplexed
optical signal. The optical amplifier can be configured to amplify the wavelength
division multiplexed optical signal to thereby produce an optically amplified wavelength
division multiplexed optical signal. The transmitter optics can be configured to accept
the optically amplified wavelength division multiplexed optical signal and transmit
an optical feeder uplink beam to the satellite in dependence thereon. The ground based
subsystem can also include a user data to spot beam controller configured to accept
a set of user data signals and combine subsets of the user data signals into the spot
beam signals that are provided to the GBBF.
[0081] In accordance with certain embodiments, the GBBF is further configured to produce
multiple copies of each of the spot beam signals accepted by the GBBF, use the phase
and amplitude coefficients to weight different copies of the spot beam signals in
different manners, and sum subsets of the weighted copies of the spot beam signals
to thereby produce the feed element signals.
[0082] The ground based subsystem can also include a plurality of frequency up-converters
each of which is configured to frequency-up convert one of the feed element signals
output by the GBBF before the feed element signal is provided to one of the EOMs.
The frequency up-converters can more specifically be configured to cause the the optical
feed element signals output by the plurality of EOMs to each have an RF frequency
within a same specified RF frequency range within which the satellite is configured
to transmit the plurality of RF service downlink beams. Additionally, the ground based
subsystem can include a plurality of filters each of which is configured to filter
one of the feed element signals after the feed element signal has been frequency up-converted,
but prior to the feed element signal being provided to one of the EOMs.
[0083] In accordance with certain embodiments, the optical feed element signals output by
the plurality of EOMs each have an RF frequency within a same specified RF frequency
range within which the satellite is configured to transmit the plurality of RF service
downlink beams.
[0084] In accordance with certain embodiments, a specified RF frequency range within which
the satellite is configured to produce and transmit the plurality of RF service downlink
beams comprises a downlink portion of the Ka band. For example, the downlink portion
of the Ka band can be from 17.7 GHz to 20.2 GHz, and thus, have a bandwidth of 2.5
GHz. For another example, the downlink portion of the Ka band can be from 17.3 GHz
to 20.2 GHz, and thus, have a bandwidth of 2.9 GHz.
[0085] In accordance with certain embodiments, an optical wavelength range of the optical
feeder uplink beam is a contiguous or non-contiguous optical wavelength range within
an infrared (IR) spectrum.
[0086] Certain embodiments of the present technology are related to a method for producing
an optical feeder uplink beam at a ground based subsystem and transmitting the optical
feeder uplink beam from the ground based subsystem to a satellite that includes a
multiple element antenna feed array and accepts the optical feeder uplink beam and
in dependence thereon produces and transmits a plurality of RF service downlink beams
to service terminals using the multiple element antenna feed array. Such a method
can include performing ground based beamforming by accepting a plurality of spot beam
signals, producing or otherwise obtaining phase and amplitude beamforming coefficients,
and producing a plurality of feed element signals in dependence on the plurality of
spot beam signals and the phase and amplitude beamforming coefficients. The method
can also include emitting a plurality of optical carrier signals each having a different
peak wavelength that is within a specified optical wavelength range, and electro-optically
modulating each of the optical carrier signals with one of the feed element signals
produced by the ground based beamforming to thereby produce a plurality of optical
feed element signals. The method can further include multiplexing the plurality of
optical feed element signals to thereby produce a wavelength division multiplexed
optical signal that includes data for the plurality of RF service downlink beams.
The method can also include producing an optical feeder uplink beam, in dependence
on the wavelength division multiplexed optical signal, and transmitting the optical
feeder uplink beam through free-space to the satellite. A method according to the
embodiments described herein can also include accepting a set of user data signals,
and combining subsets of the user data signals into the spot beam signals that are
used for the ground based beamforming.
[0087] In accordance with certain embodiments, the ground based beamforming can further
include producing multiple copies of each of the spot beam signals, using the phase
and amplitude coefficients to weight different copies of the spot beam signals in
different manners, and summing subsets of the weighted copies of the spot beam signals
to thereby produced the feed element signals.
[0088] A method can also include frequency-up converting and filtering the feed element
signals produced by the ground based beamforming prior to the electro-optically modulating.
The frequency up-converting can cause the the optical feed element signals resulting
from the electro-optically modulating to each have an RF frequency within a same specified
RF frequency range within which the satellite is configured to transmit the plurality
of RF service downlink beams.
[0089] In accordance with certain embodiments, the optical feed element signals resulting
from the electro-optically modulating each have an RF frequency within a same specified
RF frequency range within which the satellite is configured to transmit the plurality
of RF service downlink beams.
[0090] Certain embodiments of the present technology are related to a ground based subsystem
for use in transmitting an optical feeder uplink beam to a satellite that includes
a multiple element antenna feed array and that is configured to accept the optical
feeder uplink beam and in dependence thereon use the multiple element antenna feed
array to produce and transmit a plurality of RF service downlink beams to service
terminals. The ground based subsystem can include a ground based beamformer (GBBF)
including a GBBF controller, a signal replication and forward beamforming weighting
unit that is controlled by the GBBF controller, and a plurality of summers. The GBBF
controller can be configured to produce phase and amplitude beamforming coefficients
in dependence on calibration information accepted from a calibration subsystem. The
signal replication and forward beamforming weighting unit can be configured to replicate
each of a plurality of spot beam signals, weight each of the replicated spot beam
signals in dependence on phase and amplitude beamforming coefficients produced by
the GBBF controller, and output a set of phase and amplitude weighted signals. The
plurality of summers can be configured to sum respective subsets of the phase and
amplitude weighted signals to thereby produce a plurality of feed element signals.
The ground based subsystem can also include a plurality of lasers, a plurality of
electro-optical modulators (EOMs), a wavelength-division multiplexing (WDM) multiplexer,
an optical amplifier, and transmitter optics. Each of the lasers can be operable to
emit an optical signal having a different peak wavelength within a specified optical
wavelength range. Each EOM of the plurality of EOMs can be configured to accept an
optical carrier signal from a respective one of the plurality of lasers, accept a
different one of the plurality of feed element signals from the GBBF, and output a
respective optical feed element signal in dependence on the optical carrier signal
and the feed element signal accepted by the EOM. The WDM multiplexer can be configured
to accept the optical feed element signals output by the plurality of EOMs, and combine
the plurality of optical feed element signals into a wavelength division multiplexed
optical signal. The optical amplifier can be configured to amplify the wavelength
division multiplexed optical signal to thereby produce an optically amplified wavelength
division multiplexed optical signal. The transmitter optics can be configured to accept
the optically amplified wavelength division multiplexed optical signal and transmit
an optical feeder uplink beam to the satellite in dependence thereon.
[0091] The foregoing detailed description has been presented for purposes of illustration
and description. It is not intended to be exhaustive or to limit the subject matter
claimed herein to the precise form(s) disclosed. Many modifications and variations
are possible in light of the above teachings. The described embodiments were chosen
in order to best explain the principles of the disclosed technology and its practical
application to thereby enable others skilled in the art to best utilize the technology
in various embodiments and with various modifications as are suited to the particular
use contemplated. It is intended that the scope be defined by the claims appended
hereto.
1. Bodengestütztes Teilsystem (200) zur Verwendung beim Übertragen eines optischen Einspeisungs-Uplink-Strahls
(102u) zu einem Satelliten (100), der eine Antenneneinspeiseanordnung mit mehreren
Elementen einschließt und der dazu konfiguriert ist, den optischen Einspeisungs-Uplink-Strahl
(102u) zu empfangen und in Abhängigkeit davon die Antenneneinspeiseanordnung mit mehreren
Elementen zu verwenden, um eine Vielzahl von HF-Dienst-Downlink-Strahlen (106d, 110d,
114d) zu erzeugen und zu Dienstendgeräten (STs) zu übertragen, wobei das bodengestützte
Teilsystem (200) umfasst:
einen bodengestützten Strahlformer (GBBF) (230), der dazu konfiguriert ist, eine Vielzahl
von Punktstrahlsignalen anzunehmen, Phasen- und Amplituden-Strahlformungskoeffizienten
zu erzeugen oder anderweitig zu erhalten, und eine Vielzahl von Einspeiseelementsignalen
in Abhängigkeit von der Vielzahl von Punktstrahlsignalen und den Phasen- und Amplituden-Strahlformungskoeffizienten
auszugeben;
eine Vielzahl von Lasern (202), wobei jeder der Laser (202) so betrieben werden kann,
dass er ein optisches Signal sendet, das innerhalb eines spezifizierten optischen
Wellenlängenbereichs eine unterschiedliche Spitzenwellenlänge aufweist;
eine Vielzahl von elektrooptischen Modulatoren (EOMs) (204), wobei jeder EOM (204)
aus der Vielzahl von EOMs (204) dazu konfiguriert ist, ein optisches Trägersignal
von einem jeweiligen aus der Vielzahl von Lasern (202) anzunehmen, ein unterschiedliches
aus der Vielzahl von Einspeiseelementsignalen von dem GBBF (230) anzunehmen, und in
Abhängigkeit von dem optischen Trägersignal und dem Einspeiseelementsignal, die vom
EOM (204) angenommen wurden, ein jeweiliges optisches Einspeiseelementsignal auszugeben;
einen Wellenlängenmultiplex- (WDM) Multiplexer (206), der dazu konfiguriert ist, die
von der Vielzahl von EOMs (204) ausgegebenen optischen Einspeiseelementsignale anzunehmen
und die Vielzahl von optischen Einspeiseelementsignalen zu einem wellenlängengemultiplexten
optischen Signal zu kombinieren;
einen optischen Verstärker (208), der dazu konfiguriert ist, das wellenlängengemultiplexte
optische Signal zu verstärken, um dadurch ein optisch verstärktes wellenlängengemultiplextes
optisches Signal zu erzeugen; und
Übertragungsoptiken (210), die dazu konfiguriert sind, das optisch verstärkte wellenlängengemultiplexte
optische Signal anzunehmen und in Abhängigkeit davon einen optischen Einspeisungs-Uplink-Strahl
(102u) zu dem Satelliten (100) zu übertragen.
2. Teilsystem (200) nach Anspruch 1, weiter umfassend:
eine Benutzerdaten-zu-Punktstrahl-Steuerung (220), die dazu konfiguriert ist, einen
Satz Benutzerdatensignale anzunehmen und Teilsätze der Benutzerdatensignalen zu den
Punktstrahlsignalen zu kombinieren, die dem GBBF (230) bereitgestellt werden.
3. Teilsystem (200) nach Anspruch 1 oder 2, wobei der GBBF (230) weiter dazu konfiguriert
ist:
von jedem der vom GBBF (230) angenommenen Punktstrahlsignale mehrere Kopien zu erzeugen;
die Phasen- und Amplitudenkoeffizienten zu verwenden, um unterschiedliche Kopien der
Punktstrahlsignale auf unterschiedliche Arten zu gewichten; und
Teilsätze der gewichteten Kopien der Punktstrahlsignale zu summieren, um dadurch die
Einspeiseelementsignale zu erzeugen.
4. Teilsystem (200) nach einem der Ansprüche 1-3, weiter umfassend:
eine Vielzahl von Frequenz-Aufwärtswandlern (244), von denen jeder dazu konfiguriert
ist, eines der vom GBBF (230) ausgegebenen Einspeiseelementsignale frequenzmäßig aufwärts
zu wandeln, bevor das Einspeiseelementsignal einem der EOMs (204) bereitgestellt wird;
und
eine Vielzahl von Filtern (246), von denen jeder dazu konfiguriert ist, eines der
Einspeiseelementsignale zu filtern, nachdem das Einspeiseelementsignal frequenzmäßig
aufwärts gewandelt wurde, aber bevor das Einspeiseelementsignal einem der EOMs (204)
bereitgestellt wird.
5. Teilsystem (200) nach Anspruch 4, wobei die Frequenz-Aufwärtswandler (244) dazu konfiguriert
sind, die von der Vielzahl von EOMs (204) ausgegebenen optischen Einspeiseelementsignale
dazu zu bringen, dass sie jeweils eine HF-Frequenz innerhalb ein und desselben spezifizierten
HF-Frequenzbereichs aufweisen, innerhalb dem der Satellit (100) konfiguriert ist,
die Vielzahl von HF-Dienst-Downlink-Strahlen (106d, 110d, 114d) zu übertragen.
6. Teilsystem (200) nach einem der Ansprüche 1-5, wobei ein spezifizierter HF-Frequenzbereich,
innerhalb dem der Satellit (100) konfiguriert ist, die Vielzahl von HF-Dienst-Downlink-Strahlen
(106d, 110d, 114d) zu erzeugen und zu übertragen, einen Downlink-Abschnitt des Ka-Bandes
umfasst.
7. Teilsystem (200) nach Anspruch 6, wobei:
der Downlink-Abschnitt des Ka-Bandes von 17,7 Ghz bis 20,2 GHz geht und somit eine
Bandbreite von 2,5 GHz aufweist; oder
der Downlink-Abschnitt des Ka-Bandes von 17,3 GHz bis 20,2 GHz geht und somit eine
Bandbreite von 2,9 GHz aufweist.
8. Teilsystem (200) nach einem der Ansprüche 1-7, wobei ein optischer Wellenlängenbereich
des optischen Einspeisungs-Uplink-Strahls (102u) ein durchgehender oder nicht durchgehender
optischer Wellenlängenbereich innerhalb eines Infrarot- (IR) Spektrums ist.
9. Verfahren zum Erzeugen eines optischen Einspeisungs-Uplink-Strahls (102u) an einem
bodengestützten Teilsystem (200) und Übertragen des optischen Einspeisungs-Uplink-Strahls
(102u) vom bodengestützten Teilsystem (200) zu einem Satelliten (100), der eine Antenneneinspeiseanordnung
mit mehreren Elementen einschließt und der den optischen Einspeisungs-Uplink-Strahl
(102u) annimmt und in Abhängigkeit davon unter Verwendung der Antenneneinspeiseanordnung
mit mehreren Elementen eine Vielzahl von HF-Dienst-Downlink-Strahlen (106d, 110d,
114d) erzeugt und zu Dienstendgeräten (STs) überträgt, wobei das Verfahren zur Verwendung
durch das bodengestützte Teilsystem (200) umfasst:
Durchführen (602) von bodengestützter Strahlformung durch Annehmen einer Vielzahl
von Punktstrahlsignalen, Erzeugen oder anderweitiges Erhalten von Phasen- und Amplituden-Strahlformungskoeffizienten,
und Erzeugen einer Vielzahl von Einspeiseelementsignalen in Abhängigkeit von der Vielzahl
von Punktstrahlsignalen und den Phasen- und Amplituden-Strahlformungskoeffizienten;
Senden (604) einer Vielzahl von optischen Trägersignalen, die jeweils eine unterschiedliche
Spitzenwellenlänge aufweisen, die innerhalb eines spezifizierten optischen Wellenlängenbereichs
liegt;
elektrooptisches Modulieren (606) jedes der optischen Trägersignale mit einem der
durch die bodengestützte Strahlformung erzeugten Einspeiseelementsignale, um dadurch
eine Vielzahl von optischen Einspeiseelementsignalen zu erzeugen;
Multiplexen (608) der Vielzahl von optischen Einspeiseelementsignalen, um dadurch
ein wellenlängengemultiplextes optisches Signal zu erzeugen, das Daten für die Vielzahl
von HF-Dienst-Downlink-Strahlen (106d, 110d, 114d) einschließt;
Erzeugen (610) eines optischen Einspeisungs-Uplink-Strahls (102u) in Abhängigkeit
vom wellenlängengemultiplexten optischen Signal; und
Übertragen (612) des optischen Einspeisungs-Uplink-Strahls (102u) durch den freien
Raum zu dem Satelliten (100).
10. Verfahren nach Anspruch 9, weiter umfassend:
Annehmen eines Satzes von Benutzerdatensignalen; und
Kombinieren von Teilsätzen der Benutzerdatensignale zu den Punktstrahlsignalen, die
für die bodengestützte Strahlformung verwendet werden.
11. Verfahren nach Anspruch 9 oder 10, wobei die bodengestützte Strahlformung weiter umfasst:
Erzeugen (702) von mehreren Kopien jedes der Punktstrahlsignale;
Verwenden (704) der Phasen- und Amplitudenkoeffizienten, um unterschiedliche Kopien
der Punktstrahlsignale auf unterschiedliche Arten zu gewichten; und
Summieren (706) von Teilsätzen der gewichteten Kopien der Punktstrahlsignale, um dadurch
die Einspeiseelementsignale zu erzeugten.
12. Verfahren nach einem der Ansprüche 9-11, weiter umfassend:
frequenzmäßiges Aufwärtswandeln und Filtern der durch die bodengestützte Strahlformung
erzeugten Einspeiseelementsignale vor dem elektrooptischen Modulieren (606).
13. Verfahren nach Anspruch 12, wobei das frequenzmäßige Aufwärtswandeln die aus dem elektrooptischen
Modulieren (606) resultierenden optischen Einspeiseelementsignale dazu bringt, jeweils
eine HF-Frequenz innerhalb ein und desselben spezifizierten HF-Frequenzbereichs aufzuweisen,
innerhalb dem der Satellit (100) konfiguriert ist, die Vielzahl von HF-Dienst-Downlink-Strahlen
(106d, 110d, 114d) zu übertragen.